1. Field of the Invention
The invention relates to a light source device which comprises at least one so-called dielectric barrier discharge lamp. This dielectric barrier discharge lamp is a type of discharge lamp which, for example, is used as a UV light source for a photochemical reaction and in which light emitted from excimer molecules which are formed by a dielectric barrier discharge are used.
2.Description of the Related Art
As generic art with respect to the dielectric barrier discharge lamp, for example, the patent disclosure document of Japanese patent application HEI 2-7353 discloses a radiator in which a discharge vessel is filled with a discharge gas which forms excimer molecules and in which a dielectric barrier discharge forms excimer molecules; this is also called an ozonize discharge or a silent discharge, as is described in the Discharge Handbook, Electrical Engineering Society, June 1989, 7th edition, page 263, Japan. In the radiator, light is emitted from the above described excimer molecules.
The above described dielectric barrier discharge lamp and the light source device which encompasses it have various advantages which neither conventional mercury low pressure discharge lamps nor conventional high pressure arc discharge lamps have. Therefore, they have various applications. Recently, according to increasing interest in environmental pollution problems, they are finding their most important applications in materials treatment without pollution, photochemical reactions by UV rays being used. As a result, there is an extremely great demand for high power or an increase of the irradiation surface in a dielectric barrier discharge light source device.
One of the proposals which meet this demand is known, for example, from the patent disclosure document of Japanese patent application HEI 4-229671. Here, it is described that, by parallel luminous operation of several dielectric barrier discharge lamps, the light source and the irradiation surface are enlarged. However, major problems arise which are not resolved by the prior art alone, i.e., simultaneously achieving uniformity of the irradiation energy density and the possibility of light control in the case of irradiation of a large surface.
The action of material treatment by UV rays of the dielectric barrier discharge lamp is based on extremely complex, highly developed photochemical reactions. To obtain the desired effect of material treatment in a material with a large surface (area), the irradiation energy density distribution should be neither too great or too small compared to the desired distribution. It is apparent that an overly low irradiation energy density is considered a disadvantage because the irradiation effect is too little. In the case of an excess irradiation energy density, there are cases in which, for example, repeated reactions of the decomposition products by the irradiated UV rays, and thus unintended molecular syntheses take place, and in which nonuniform layers of impurities are formed on the surface of the material to be treated. The irradiation energy density should, therefore, not be too large or too small above and beyond an allowable range which depends on the reaction types of the material treatment to be conducted. An ideal dielectric barrier discharge device must, therefore, have uniformity of the irradiation energy density and the control function for the irradiation energy density which satisfy this condition.
There are two processes for control of the irradiation energy density, i.e., a process in which the irradiation duration is controlled, and one in which the amount of radiated light of the lamp is controlled, i.e., light control.
The first process is accomplished extremely simply. However, here it is considered a disadvantage that this process can only be used for a material to be irradiated which can be treated flat and item-by-item, for example, a glass plate. However, in the case of a roll-like sheet material which flows and is transported continuously, the second, light control, process is needed.
In the following, using FIG. 1, it is described why it is difficult to achieve uniformity of the irradiation energy density in a dielectric barrier discharge device while enabling light control at the same time.
In luminous operation of the dielectric barrier discharge lamp, an AC voltage with a high frequency of, for example, 10 kHz to 200 kHz and 2 kV to 10 kVrms is applied to the electrodes at both of its poles. In dielectric barrier discharge lamps B1, B2, however, between the electrodes Ea1, Eb1, and Ea2, Eb2, there is, respectively, a space G1 and G2 for the discharge plasmas and one or two layers of a dielectric D1, D2 which act as a capacitor. In this way, current flows. When current flows, the discharge plasmas here can be roughly regarded as a resistor. The voltage applied to the discharge plasmas, however, repeats starting of the discharge and stopping of the discharge in each half period, because it is an AC voltage.
The dielectric barrier discharge lamps are, therefore, an essentially nonlinear elements. For example, a dielectric barrier discharge lamp with an electrode surface of 200 cm.sup.2 in which the distance between two quartz glass plates with a thickness of 1 mm was 4 mm, and in which this distance was filled with xenon gas with a pressure of roughly 40,000 Pa as the discharge gap, was operated with a frequency of roughly 100 kHz and an applied voltage of roughly 4 kVrms in measurement tests of the inventors, and the result was obtained that this corresponds roughly equivalently to a series-connected arrangement of a capacitor of roughly 200 pF with a resistor of roughly 1.5 kiloohm.
With respect to lamp production, in the process of material procurement for this purpose, or in the production process, there are always processing faults and variations. The ignition characteristic of the lamps is different for each lamp. For example, if quartz glass is used as the dielectric D of the dielectric barrier discharge lamp, in economically procurable quartz glass with a nominal thickness of 1 mm, there are thickness variations of roughly 0.3 mm and also local nonuniformities to roughly the same degree. As a result of faults in processing, variations between the two quartz glass plates furthermore occur. In addition, also, within a single lamp, there is local nonuniformity of the distances when the quartz plates have a distorted shape.
These variations and nonuniformities exert major effects on the ignition voltage and the discharge maintaining voltage of the dielectric barrier discharge lamp which occur as nonuniformities of the ignition voltage and the discharge maintenance voltage in each lamp or in places within a single lamp. The nonuniformities of the ignition voltage and the discharge maintaining voltage cause nonuniformities of the emission intensity in each lamp and each pertinent location within the lamp.
These influences occur especially distinctly in the case of a relatively low peak value of the voltage applied to the lamp. The reason for this is quite apparent if, for example, a state is assumed in which the peak value of the voltage applied to the lamp is roughly as large as the average discharge maintaining voltage, and if it is considered that there must be sites where a discharge takes place and that, on the other hand, there must also be sites where no discharge at all takes place. FIG. 1 schematically shows that there are lamps with a long discharge gap and lamps with a short discharge gaap. In this case, in the lamp with the long discharge gap the ignition voltage is high, this makes it difficult to accomplish the discharge.
FIG. 2(a1) and FIG. 2(a2) schematically show the relation between ignition voltage level Ve and voltage V applied to the lamp in this state. FIG. 2(a1) shows a lamp or parts within a lamp with a high ignition voltage level. FIG. 2(a2) shows a lamp or parts within a lamp with a low ignition voltage level. Voltage V applied to the lamp is the same in both representations. In FIG. 2(a1) applied AC voltage V does not reach ignition voltage level Ve. In doing so, therefore, no discharge at all takes place. On the other hand, in FIG. 2(a2), there is a time interval Pe in which the applied AC voltage V rises above the ignition voltage level. In this time interval, a discharge takes place.
Conversely, in the state in which the peak value of the voltage applied to the lamp as compared to the average discharge maintaining voltage is relatively higher, it can be stated that the nonuniformities of the discharge state in each lamp or within a single lamp become less.
FIG. 2(b1) and FIG. 2(b2) schematically show, in contrast to FIG. 2(a1) and FIG. 2(a2), the relation between ignition voltage level Ve and voltage V applied to the lamp in this state. Voltage V applied to the lamp is the same in both representations, but is higher than in FIG. 2(a1) and FIG. 2(a2). In this case, there is a time interval Pe in the two representations, in which the applied AC voltage V rises above the ignition voltage level. It can be intuitively understood that an increase of the voltage V applied to the lamp results only in a relatively small prolongation of the time intervals.
The discharge maintaning voltage is less than the ignition voltage. The timing of the end of discharge is therefore shifted rearwardly towards the end of time interval Pe in FIG. 2(a2), FIG. 2(b1) and FIG. 2(b2), in which an increase above the ignition voltage level takes place. But, this is not fundamentally important.
However, if the voltage applied to the lamp is fixed only to obtain the required uniformity, the lamp power cannot be fixed to less than or equal to a certain value, by which light control can no longer be performed. Furthermore, depending on the lamp power value, there is also the danger of premature breakage due to excess power. A simple increase of the voltage applied to the lamp is, therefore, not a significant solution of the problems.